Question

One post-war fact about the United States that created tension wit the USSR was the United State

Answer 1

One big factor in the tension:  The United States had atomic bombs, while the Soviet Union did not.

• The USA  would not share its weapons technology with the Soviets, who had been an ally of the US in World War II.  By 1949, the Soviets had developed and tested their own atomic bomb, which led to an arms race between the two powers.

Another factor:

• Tensions over the East Germany / West Germany and East Berlin / West Berlin division of territory.  The Soviets tried to impose a blockade of Berlin, and the US responded with the Berlin Airlift to get supplies into West Berlin.

Those were two specific factors.  Underneath it all was a bigger, philosophical issue:  The USA was committed to capitalism and democratic institutions of government.  The USSR was committed to communism and imposed authoritarian government.  The Cold War was mostly a tension between these worldviews.